1. Field of the Invention
The present invention relates to radio frequency identification (RFID) systems. More particularly, the invention relates to a tag containing an RFID transponder having an antenna that can be adjusted or tuned to achieve optimal performance characteristics.
2. Description of Related Art
Radio frequency transponders are used in many applications. In the automatic data identification industry, the use of RFID transponders (also known as RF or RFID tags) has grown in prominence as a way to obtain data regarding an object onto which an RF tag is affixed. An RF tag generally includes a memory in which information may be stored. An interrogator containing a transmitter-receiver unit is used to query an RF tag that may be at a distance from the interrogator and moving relative to the interrogator. The RF tag detects the interrogating signal and transmits a response signal containing encoded data back to the interrogator. Such RF tags may have a memory capacity of several kilobytes or more, which is substantially greater than the maximum amount of data that may be contained in a bar code symbol or other types of human-readable indicia. Further, the RF tag memory may be re-written with new or additional data, which would not be possible with a printed bar code symbol. RF tags may also be readable at a distance without requiring a direct line-of-sight view by the interrogator, unlike bar code symbols or other types of human-readable indicia that must be within a direct line-of-sight and which may be rendered entirely unreadable if obscured or damaged. The RF tags may either extract their power from the RF interrogating field provided by the interrogator, or may include their own internal power source (e.g., battery).
More particularly, an RF tag includes a semiconductor chip containing RF circuitry, control logic, and memory. The semiconductor chip may be mounted on a substrate that also includes an antenna. In some applications, RF tags are manufactured by mounting the individual elements to a circuit card made of epoxy-fiberglass composition or ceramic. The antennas are generally loops of wire soldered to the circuit card or consist of metal etched or plated onto the circuit card. The whole assembly may be encapsulated, such as by enclosing the circuit card in a plastic box or molded into a three dimensional plastic package. Recently, thin flexible substrates such as polyamid have been used to reduce the size of the RF tag in order to increase the number and type of applications to which they may be utilized.
With respect to RF tags that draw their power from the interrogating RF field, it is necessary that the antenna connected to the front end of the RF circuitry produce an output voltage that is above a particular threshold voltage. These front end circuits rectify the RF carrier component of the modulated electromagnetic field that excites the antenna, leaving the modulated signal (i.e., envelope) at the output of the front end. To optimize the voltage and/or power produced for the RF tag, there must be a good impedance match between the antenna and the front end of the RF circuitry at the resonance frequency. If the impedance match of the antenna/front end combination is not optimal, the RF tag will have a limited range (i.e., distance) over which it can communicate. There are many known ways to match the antenna and front end impedances, such as impedance matching circuits using discrete components, e.g., inductor/capacitor networks, or distributed elements such as microstrip structures. These known impedance matching techniques are not desirable for many RF applications, however, since they tend to increase the cost, complexity and size of the RF tags.
It is also known to provide the antenna, preferably a dipole antenna, with one or more loading bars that are placed adjacent to the elements of the antenna at a spacing distance. By adjusting the loading bar length, width, and/or spacing distance and/or the number of loading bars, the real part of the antenna input impedance can be changed. Alternatively, one or more stubs may be added to one or more of the antenna elements. The stubs act as two-conductor transmission line that is terminated either in a short-circuit or open-circuit. The short-circuited stub(s) acts as a lumped inductor (capacitor) when the length of the transmission line is within odd (even) multiples of one quarter xe2x80x9cguided wavelengthxe2x80x9d of the transmission line. The guided wavelength has a known relation to the wavelength to which the antenna is tuned. The open-circuited stub(s) acts as a lumped capacitor (inductor) when the length of the transmission line is within odd (even) multiples of one quarter of the guided wavelength. The magnitude of these lumped capacitors and inductors (reactances) is affected not only by the material surrounding the stub, but also is affected by a stub length, a stub conductor width, and a stub conductor spacing. Zero or more short-circuit stubs and zero or more open-circuit stubs are added to one or more of the antenna elements to change the reactive (imaginary) part of the antenna input impedance. The reactive part of the antenna input impedance could be changed to equal the negative magnitude of the reactive part of the front end input impedance. This gives the maximum voltage for a given real part (Ra) of the antenna input impedance and the maximum power transfer between the antenna and the front end.
A drawback with these and other impedance matching techniques for RF tags is that they cannot be adjusted after manufacture to account for minor variations in input impedance that result from manufacturing variations and other factors. The antenna impedance may be affected by the dielectric constant of the substrate material to which the antenna is affixed. Further, the semiconductor chip providing the RFID circuitry may have slight variations in manufacture tolerances that affect the input impedance of the RF front end. These and other factors result in impedance mismatches that reduce the performance of the RF tag. The loading bars and stubs are generally designed for average expected input impedance of the RF circuitry, and cannot be altered after the impedance matching elements are formed on the substrate.
Accordingly, it would be very desirable to provide an RF tag having an antenna that can be adjusted or tuned to achieve optimal performance characteristics.
The present invention provides an RF tag having an antenna that can be selectively tuned to achieve optimal performance. The tuning can occur during the manufacturing process so that the RFID circuitry has an optimum impedance match with the antenna.
More particularly, the RF tag comprises an RF transponder integrated circuit and an antenna connected to the RF transponder integrated circuit. The antenna includes components such as tuning stubs and loading bars that are physically alterable to selectively vary the performance characteristics of the antenna. The tuning stub and loading bar may each further comprising a variably selectable length having elements that may be removed by punching, cutting, etching, laser trimming or other process. The antenna may further comprise a leadframe or a flexible substrate.
The invention further provides a method of tuning an antenna for an RF transponder. A test signal is sent to an antenna of an initial size and dimension. The strength of a radiated signal received from the antenna is measured, and at least one physical characteristic of the antenna is adjusted. These measuring and adjusting steps may be repeated until optimum strength of the radiated signal is obtained. The antenna further comprises components that are physically alterable to selectively vary performance characteristics of the antenna, and the adjusting step further comprises adjusting dimensions of the physically alterable components. More specifically, the adjusting step further comprises removing a portion of a tuning stub of the antenna in order to alter impedance of the antenna, and/or removing a portion of a loading bar of the antenna in order to alter impedance of the antenna.
In another embodiment of the invention, an apparatus for tuning RF transponders is provided. The tuning apparatus includes a transport mechanism having a supply reel and a take-up reel adapted to transport a tape along a transport path extending therebetween. The tape has a plurality of RF transponder antennas formed thereon. The antennas further comprise respective components that are physically alterable to selectively vary an impedance characteristic of the antennas. A punching tool is disposed on the path between the supply reel and the take-up reel. The punching tool is selectively movable laterally with respect to the tape to execute a punching operation in a portion of the tape corresponding with one of the physically alterable components. Each of the RF transponder antennas of the tape can be adapted to have unique impedance characteristics.
A more complete understanding of the adjustable length antenna system for RF tags will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.